The television system in use in the United States today is based on the National Television Systems Committee (NTSC) standard. This standard was adopted in 1953 and has remained unchanged since then. Although the standard is used in many foreign countries, it has limitations that newer standards seek to avoid. Thus, additional standards, such as Phase Alternate Line (PAL), Sequential Color and Memory (SECAM), and Multiplexed Analog Component (MAC) have been developed and used outside the United States.
At the receiving end of the television system, one major limitation of the NTSC standard signal is crosstalk between demodulator output signals, which causes color distortion. In addition the NTSC system is sensitive to transmission path differences that introduce phase errors, which cause color errors. It is this latter limitation of NTSC systems that the PAL standard seeks to improve. PAL signals reverse the phase of the color information on adjacent horizontal lines so that phase errors may be averaged out by the viewer's eyes.
Both the NTSC and the PAL systems use composite television signals, containing a luminance signal and two chrominance signals modulated on a color subcarrier. The color subcarrier is both amplitude and phase modulated by the color signals. Another type of composite system is the SECAM system, in which color transmission is in a line sequential form, with only one of two color difference signals transmitted at one time. First, one color difference signal is transmitted on one line, then the other color difference signal is transmitted on the following line. Two frequency modulated subcarriers are used to represent the color difference signals. As compared to composite systems, the MAC system is a component system, which time-multiplexes chrominance and luminance signals.
A recent advance in consumer electronics is in the area of improved television pictures using these standard signal transmissions. Because existing transmission standards are deeply entrenched and difficult to change, one approach would be to re-design television receivers to improve the display produced by standard signal transmissions. This approach is referred to as improved definition television (IDTV).
IDTV systems would have many advantages over analog systems. The picture improvements would include non-interlacing, filtering, and noise reduction. From the viewer's standpoint the effect should be reduction of artifacts such as line crawl, line structure visibility, line flicker, large area flicker, and picture interference. IDTV systems should also reduce cross luminance and cross chrominance and clean noise from weak signals.
A need exists for a digital processor system for producing IDTV pictures, which operates in real time and minimizes overhead from memory/processor input and output. Ideally, the system should be software programmable so that processing functions can be developed and tested before the system is permanently programmed.
The use of digital processors and field memory devices enables special picture features, which are not available with analog television receivers. These special features would include multiple screen displays and still pictures. Thus, a need also exists for a digital television receiver system that provides special features of this nature.